Fuel injection rate shaping is a process of tailoring the initial portion of fuel delivery to control the amount of fuel delivered during the ignition delay portion and the main injection portion of an injection cycle. This process modifies the heat release characteristics of the combustion process and is beneficial in achieving low emission and noise levels.
Rate shaping devices for conventional mechanically-driven fuel systems may be classified as one of three types: restrictive, retractive, or spill control. Restrictive devices cause a pressure drop of the injection fuel, resulting in lower injection pressure. Retractive devices temporarily store fuel during the initial portion of injection, while spill control devices spill or bleed a portion of the fuel flow from the high pressure fuel injection circuit.
Known spill control devices, associated with a fuel pump plunger reciprocated in synchronism with rotation of an engine, are shown in U.S. application Ser. No. 356,818 by Links et al. published on Jun. 1, 1943, U.S. Pat. No. 2,547,174 issued to Rogers on Apr. 3, 1951, U.S. Pat. No. 3,792,692 issued to Kiley on Feb. 19, 1974, and U.S. Pat. No. 5,020,979 issued to Askew on Jun. 4, 1991. In such devices, the flow of fuel sprayed by a fuel injection nozzle is temporarily interrupted or reduced by spilling a portion of fuel from a high pressure plunger chamber. The fuel is spilled when a certain control edge or port on the reciprocal plunger communicates with a certain control edge or port in a stationary barrel or housing surrounding the plunger.
Although the general effects of rate shaping are well known, the trade-offs with overall engine performance are not generally known. This is partly due to the fact that, with a mechanically-driven fuel pump plunger, the fuel system rate shaping characteristics are limited in operating range, that is, they are dependent on engine load and speed. An engine operating over a large speed range will likewise vary the speed of the mechanically-driven fuel pump plunger. Consequently, there is usually only one satisfactory engine operating condition at which the rate shaping device provides satisfactory or optimum benefits.
U.S. Pat. No. 5,121,730 issued to Ausman et al. on Jun. 16, 1992 shows a hydraulically-actuated fuel injector system in which the fuel pump plunger of each injector is hydraulically actuated or driven by a source of pressurized actuating fluid. This system has some inherent injection rate shaping capability compared to a mechanically-actuated injector having no rate shaping device at all. However, there are times when the amount of injection rate shaping provided in Ausman et al. is lower than desired.
U.S. Pat. No. 4,878,471 issued to Fuchs on Nov. 7, 1989 shows a hydraulically-actuated fuel injector system having an injection rate shaping device. This injection rate shaping device adds a number of components to the basic fuel injection system which undesirably adds complexity and cost. Fuchs requires fluid pressure feedback from the high pressure fuel plunger chamber to control the amount of actuating fluid pressure available to stroke the plunger. Fuchs also requires a pair of valves in the hydraulic control unit for controlling the flow of actuating fluid used to hydraulically actuate the fuel pump plunger associated with each injector. Fuchs also requires a control unit associated with the high pressure fuel injection circuit of the injector.
The present invention is directed to overcoming one or more of the problems as set forth above.